The present invention relates to the field of power plant technology. It relates to a method for setting or regulating the steam temperature of the live steam and/or reheater steam, particularly under part load, in a combined-cycle power plant and to a combined-cycle power plant for carrying out the method.
Power plant concepts, in which gas turbine sets having downstream heat recovery steam generators and water/steam cycles having fired boilers and corresponding steam turbines are combined with one another, have been known for some time from the prior art and are used increasingly, above all because of the increased overall efficiency, the additional power output (gas turbine set), along with low investment, and operating flexibility.
A variant of such combined power plant concepts is the combined-cycle power plant, in which the gas turbine set and the water/steam cycle are coupled to one another on the water/steam side. The flue gas discharged by the gas turbine set is conducted through a heat recovery steam generator for the generation of steam and is then discharged into the surroundings. The steam generated in the heat recovery steam generator is fed, for example as live steam and/or reheater steam, into the water/steam cycle at suitable points. Additionally or alternatively, the heat recovery steam generator may also be employed for condensate and/or feedwater preheating. Various types of combined-cycle power plants with different connections between the gas turbine system and the water/steam system are described, for example, in the article by G. Bauer et al., xe2x80x9cDas Verbundkraftwerkxe2x80x94eine neue Variante des kombinierten Kraftwerkesxe2x80x9d, [xe2x80x9cThe combined-cycle power plantxe2x80x94a new variant of the combined power plantxe2x80x9d] VGB Kraftwerkstechnik 73 (1993), No. 2, page 120 ff.
Connection variants are in this case (1) the generation of live steam in the heat recovery steam generator, (2) the generation of reheater steam or medium-pressure steam in the heat recovery steam generator, (3) the generation of low-pressure steam in the heat recovery steam generator, (4) reheating in the heat recovery steam generator or (5) a combination of the first four connection variants with branch flows.
In combined-cycle power plants, essential importance is attached to keeping constant or regulating the live steam temperature, particularly with a view to operation under part load.
In the conventional steam generator, the flue gas temperature falls under part load. As a result of this, the live steam temperature cannot be maintained. However, a fall in the live steam temperature signifies a lowering of the efficiency of the cyclic process. If the steam temperature is to be capable of being regulated over a relatively wide power output range, water injection, together with a corresponding system design, may be provided in a known way. However, this signifies an undesirable loss of energy in the steam generator.
In the gas turbine set, the flue gas parameters depend on the load of the gas turbine and on the climatic conditions. This results in influence being exerted on the generation of steam in the heat recovery steam generator.
Overall, satisfactory part-load properties of a conventional steam generator cannot be achieved in a combined-cycle power plant by means of the above-mentioned known methods. A good part-load behavior is of great importance, particularly when the power plant is used in the medium-load range.
An object of the invention is, therefore, to provide a method for setting or regulating the steam temperature of the live steam and/or reheater steam, particularly under part load, in a combined-cycle power plant, which leads to markedly improved part-load properties of the combined-cycle power plant, and a combined-cycle power plant for carrying out such a method.
The essence of the invention is to set or regulate the steam temperature of the live steam or reheater steam flowing to the steam turbine by admixing steam from the heat recovery steam generator with this steam, and by setting or regulating the steam temperature of this admixed steam. Improved part-load properties of the combined-cycle power plant can thereby be achieved in a simple way.
A first refinement of the method according to the invention is characterized in that, in order to set or regulate the steam temperature of the steam generated in the heat recovery steam generator, a corresponding change in the feedwater mass flow flowing through the heat recovery steam generator for steam generation is carried out. For this purpose, the feedwater mass flow may be controlled, for example, by the associated feedwater pump, as is known, for example from U.S. Pat. No. 5,237,816.
A second refinement of the method according to the invention is distinguished in that a heat recovery steam generator is used which comprises an evaporator and a downstream superheater and in which a separator is arranged between the evaporator and the superheater. The use of the separator limits the setting and regulating range of the steam temperature toward low temperatures.
The steam generated in the heat recovery steam generator may, within the scope of the invention, be fed into the water/steam cycle at different points:
One possibility is for the fired boiler to be followed by a superheater, for live steam to be generated in the heat recovery steam generator and for the live steam generated in the heat recovery steam generator to be fed into the water/steam cycle downstream of the superheater
Another possibility is for the fired boiler to be followed by a superheater with a plurality of successively arranged heat transfer surfaces, for live steam to be generated in the heat recovery steam generator and for the live steam generated in the heat recovery steam generator to be fed into the water/steam cycle between the heat transfer surfaces of the superheater.
Another possibility is for the steam turbine to comprise a high-pressure stage and a medium-pressure stage, for a reheater to be provided between the highpressure stage and the medium-pressure stage, for reheater steam to be generated in the heat recovery steam generator and for the reheater steam generated in the heat recovery steam generator to be fed into the water/steam cycle downstream of the reheater.
It is possible, furthermore, for the steam turbine to comprise a high-pressure stage and a medium-pressure stage, for a reheater with a plurality of successively arranged heat transfer surfaces to be provided between the high-pressure stage and the medium-pressure stage, for reheater steam to be generated in the heat recovery steam generator and for the reheater steam generated in the heat recovery steam generator to be fed into the water/steam cycle between the heat transfer surfaces of the reheater.
The combined-cycle power plant according to the invention for carrying out the method, which combined-cycle power plant comprises a water/steam cycle, in particular with a steam turbine, with a fired boiler and with means for superheating or reheating the steam generated in the boiler to form live steam or reheater steam, and also a gas turbine set with a downstream heat recovery steam generator, the heat recovery steam generator being connected to the water/steam cycle in such a way that steam generated in the heat recovery steam generator is admixed with the live steam or reheater steam, is characterized in that the means for superheating or reheating have a plurality of successively connected heat transfer surfaces, and in that the steam generated in the heat recovery steam generator is fed into the water/steam cycle between the heat transfer surfaces.
Further refinements of the invention may be gathered from the dependent claims.